Aldehydes with Pyridinium Chromates from demorol, HTML by metanoid
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Acid chlorides can be easily converted to aldehydes by reductive oxidation using LAH and Pyridinum chlorochromate (PCC) or Pyridinum dichromate (PDC). Since this procedure uses no exotic or watched chemicals and since yields are pretty good, this might be another way to our beloved aldehydes.1
Experimental section
All reaction were performed under a dry N2 atmosphere. All chemicals used were commercial products of the highest purity available; THF was dried over 4 A molecular sieve and distilled from sodium-benzophenone ketyl prior to use. Methylene chloride was also dried over P4O10 and distilled. 1H NMR spectra were recorded on a Bruker AMX 300 spectrometer. Gas chromatographic analyses were carried out with a Varian 3300 Chromatograph.
Reductive Oxidation of Acid Chlorides to Aldehydes
The following reaction is typical of the procedure utilized in such conversion with PCC. A solution of lithium aluminum hydride (1.0 M, 31mL, 31 mmol) in THF was placed in an oven-dried, 500-mL flask fitted with a side-arm and a reflux condenser leading to a mercury bubbler. To this solution 8.57g (61 mmol) of benzoyl chloride was added dropwise with vigorous stirring at 0°C and the mixture was stirred further for 3 hours at room temperature. To a well stirred solution of PCC (14.5g, 67 mmol) in methylene chloride (120mL) taken in a 500-mL flask equipped as described above, was added dropwise the above reaction mixture in THF using a cannula. The mixture was stirred for 3 hours at room temperature. GC analysis of aliquot using tridecane as an internal standard indicated a yield of 98%.
Isolation of Aldehyde Products
The rest of reaction mixture (60 mmol) was diluted with ethyl ether (120mL) and the supernatant liquid is then filtered through Florisil® (120g) contained in a 300-mL sintered glass funnel. The solid residue was triturated with ethyl ether (3 × 30mL) and passed through the same Florisil column. The filtrate is concentrated and distilled under reduced pressure to give 5.2g (82%) of pure benzaldehyde, bp 62-63°C (14 mmHg, np22 1.5450).
color=»green»> Chloride Aldehyde Yield (PCC) % Yield (PDC) % Reaction Time (h) Benzoyl chloride Benzaldehyde 82 79 3 4-Chlorobenzoyl chloride 4-Chlorobenzaldehyde 97 97 3 o-Toluoyl chloride o-Tolualdehyde 96 95 3 p-Toluoyl chloride p-tolualdehyde 95 95 3 o-Anisoyl chloride o-Anisaldehyde 96 94 3 p-Anisoyl chloride p-Anisaldehyde 96 95 3
Conversion of Alcohols to Aldehydes and Ketones by Oxidation of Trialkoxyaluminum with Pyridinum Chlorochromate (PCC) color=»green»>2
Experimental procedure
The following experimental procedure is illustrative. To an oven-dried, nitrogen-flushed 100-mL RB flask, fitted with a septum inlet, a magnetic stirring bar, and a reflux condenser leading to a mercury bubbler, a 1.10 M solution of aluminum hydride (18.2mL, 20 mmol) in THF was injected and the solution was kept at 0°C with aid of an ice-water bath. The mixture was stirred at 0°C and a 3.0 M solution of 1-octanol (20mL, 60 mmol) was added dropwise with a syringe. After the addition was complete, the mixture was then allowed to room temperature and stirred for 3 h. In another oven-dried, nitrogen-flushed 500-mL RB flask, fitted with a septum inlet, a magnetic stirring bar, and reflux condenser leading to a mercury bubbler, are placed powdered PCC (26g, 120 mmol) and methylene chloride (200mL). To the well-stirred suspension, a solution of trioctylaluminum in THF thus prepared was added with the aid of a double-ended needle. The mixture was stirred at room temperature for 1 h. Then, ethyl ether (200mL) was added and the mixture was filtered through a column containing Florisil®. The solid residue in the flask was triturated with ethyl ether (3 × 50mL) and filtered through the same Florisil column. The combined filtrate was concentrated and distilled to afford 6.04g of pure octanal (78%); bp 170-172°C/761 mmHg. The purity was further confirmed by GC analysis.
A small scale of same reaction (trioctylaluminum, 1 mmol) was also performed and tridecane was added as an internal standard. The product aldehyde was analyzed by GC with use of a Carbowax TAP capillary column (25 m) to show 97% octanal formation.
Alkyl group of (RO)3Al Product Yield (%) Ph-CH2- BA* 98 4-Me-Ph-CH2- 4-Me-BA 97 4-MeO-Ph-CH2- 4-MeO-BA 97 4-Cl-Ph-CH2- 4-Cl-BA 98
*BA = Benzaldehyde
Conversion of Carboxylic Acids into Aldehydes by Oxidation of Alkoxyaluminum Intermediate with Pyridinum Chlorochromate or Pyridinum Dichromatecolor=»green»>3
Experimental procedure
This method provides another convenient procedure for the direct conversion of carboxylic acids to corresponding aldehydes. The following procedure for the reaction of hexanoic acid is representative. An oven-dried, 250-mL RB flask with sidearm, equipped with a magnetic stirring bar and a reflux condenser, was attached to a mercury bubbler. The flask was flushed with dry nitrogen and then maintained under a static pressure of nitrogen. The flask was charged with hexanoic acid (6.97g, 60 mmol) and 30mL of THF. The flask was immersed in an ice-water bath and a pre-cooled 1.0 M solution of aluminum hydride (30mL, 30 mmol) in THF was added dropwise with vigorous stirring. After the complete evolution of the hydrogen, the ice-water bath was removed and the reaction mixture was stirred for 30 min at room temperature.
To a well-stirred suspension of PCC (14.3g, 66 mmol) in methylene chloride (100mL) taken in a 500-mL RB flask equipped as described above, is added dropwise the above solution of alkoxyaluminum intermediate in THF using a cannula. The mixture was stirred for 12 h at room temperature. The small portion of this mixture was transfered to a vial and dodecane was added as an internal standard. GC analysis using a Carbowax 20 M capillary column (20 m) showed a presence of hexanal in a yield of 96%. The reaction mixture was diluted with 200mL of diethyl ether and the supernatant liquid is filtered through Florisil® (100g) contained in a 300-mL sintered glass funnel. The solid residue is washed with diethyl ether (3 × 50mL) and passed through the same Florisil column. The filtrate was concentrated and distilled to afford pure hexanal (4.93g, 82% yield); bp 129-130°C (754 mmHg).
Analogous procedures are used for the synthesis of the other aldehydes listed in Table 1. In the case of PDC as an oxidant used, actually the same procedure was adopted except the oxidation time.
Acid Aldehyde Yield % (PCC) Yield % (PDC) Reaction time (h) Cinnamic acid Cinnamaldehyde 95 98% 12 Benzoic acid Benzaldehyde 85%* 99% 6 o-Toluic acid o-Tolualdehyde 99% 99% 6 m-Toluic acid m-Tolualdehyde 98% 99% 6 p-Toluic acid p-Tolualdehyde 99% 99% 6 p-Anisic acid p-Anisaldehyde 98% 98% 6 4-Chlorobenzoic acid 4-Chlorobenzaldehyde 99% 98% 6
*Isolated yield
Convenient Conversion of Carboxylic Esters to Aldehydes by Oxidation of Alkoxyaluminum Intermediate with Pyridinum Chlorochromate or Pyridinum Dichromatecolor=»green»> 4
This method provides another convenient procedure, which is superior to many of the procedures previously utilized.
Experimental procedure
The following procedure for the reaction of ethyl benzoate with PCC is representative. An oven-dried, 250-mL RB flask, fitted with a side arm and a reflux condenser connected to a mercury bubbler, was flushed with dry nitrogen and then maintained under a static pressure of nitrogen. The flask was charged with 1.0 M solution of aluminum hydride (61mL, 61 mmol) in THF. To the stirred solution at room temperature, ethyl benzoate (9.61g, 61 mmol) was added dropwise and the reaction mixture was stirred for 1 hour. To a well stirred suspension of PCC (28.5g, 132 mmol) in methylene chloride (200mL) taken in a 500-mL flask equipped as described above, was added dropwise to the above solution of alkoxyaluminum intermediate in THF using a cannula. The mixture was stirred for 3 h at room temperature. The small portion of this mixture was transferred to a vial and dodecane was added as an internal standard. GC using a capillary column of Carbowax 20 M indicated the presence of benzaldehyde in a yield of 99%.
The rest of the reaction mixture (60 mmol) was diluted with diethyl ether (200mL). The supernatant liquid was filtered through Florisil® (200g) contained in a 300-mL sintered glass funnel; the solid was triturated with ethyl ether (3 × 50mL) and passed through the same column. The filtrate was concentrated and distilled under reduced pressure to give pure benzaldehyde (5.22g, 82%), bp 62-63° C (15 mmHg).
Analogous procedure is used for the synthesis of the other aldehydes. In the case of PDC as an oxidant used, actually the same procedure was adopted.
Ester Aldehyde Yield % (PCC) Yield % (PDC) Reaction time (h) Ethyl butyrate Butyraldehyde 94 93 6 Ethyl cinnamate Cinnamaldehyde 96 95 3 Methyl benzoate Benzaldehyde 99 98 3 Ethyl benzoate Benzaldehyde 82 81 3 Ethyl 4-methylbenzoate 4-methylbenzaldehyde 97 98 3 Methyl 4-chlorobenzoate 4-chlorobenzaldehyde 98 96 3
References
[1]Bull. Korean Chem. Soc. 2000, Vol. 21, No. 4, pp 375
[2]Bull. Korean Chem. Soc., 1998, Vol. 19, No.7, pp 724
[3]Bull. Korean Chem. Soc., 1998, Vol. 19, No.7, pp 730
[4]Bull. Korean Chem. Soc., 1998, Vol.19, No. 12, pp 1301
